A conventional method of improving the processing properties consists in passing the strip through an annealing furnace under a certain tensile stress.
A known problem with this method consists in the fact that the preceding section of the production of the steel strip may cause defects in the edges of the strips, which defects may lead to the strip breaking in the furnace when it is under stress.
Continuous annealing furnaces are highly complicated installations which require a very considerable time to thread the strip through. If a strip breaks, this loss of time causes enormous production losses. It is therefore essential to limit the occasions when the strip breaks to an absolute minimum.
A conventional way of achieving this consists in detecting defects in the edge of the strip in the feed path of the metal strip which precedes the annealing section. An opto-electronic system is often used to do this, in which case a defect detected by a camera is translated into an electronic signal.
It should be noted that defects may comprise hairline cracks running from the edge of the strip, but also loose flaps at the edge or relatively coarse inclusions. Sudden changes in the width of the strip may sometimes also be regarded as defects. A defect causes a local increase in stresses in the strip if the latter is under stress. This increase in stresses may initiate a crack which results in the strip breaking.
The most simple measures which follow the detection of a defect consist in the movement of the strip being stopped and in a piece being removed from the edge of the strip at the location of the defect. Although such a measure prevents the strip from breaking, it nevertheless leads to a loss of production, while furthermore a piece of strip has been subjected to a different heat treatment, which may also lead to differences in quality of the annealed strip.